This change in the field of history can generally be associated with the upheaval of the 1960s.
In the 1960s, the counterculture arose, challenging “establishment” ideas. This was also a time when various minority groups as well as women pushed for rights. In this context, historical thinking changed. Many historians wanted to stop focusing on the doings of the “establishment” elites, which is what political history is. Instead, they wanted to focus on the “little people.” They wanted to look at how regular people experienced various historical times and events.
This switch, then, came out of a change in social values during the 1960s.